Photodiodes are reverse biased to form a depleted semiconductor region with a high electric field that serves to separate photogenerated electron-hole pairs. A photodiode is operated at a reverse bias voltage that is high enough for electron avalanche multiplication to take place wherein electron-hole pairs are generated by the impact ionization process, giving rise to internal current gains. As pointed out by S. N. Sze in Physics of Semiconductor Devices, 2d Ed. John Wylie & Sons, 1981, pp. 766-783, incorporated herein by reference, there are a number of limitations that have been placed on such devices resulting from inherent deficiencies. For example, for high-speed operation, it is desired to keep the depletion region thin, so as to reduce transit time, but, at least for infrared or near infrared absorption, in order to increase quantum efficiency (defined as the number of electron-hole pairs generated per incident photon), the depletion layer must be sufficiently thick to allow a large fraction of the incident light to be absorbed. Sze also points out that an avalanche photodiode requires the avalanche multiplication to be spatially uniform over the entire light-sensitive area of the diode. Of most importance here are "non-uniformities" caused by resistivity fluctuations and variations. Also, microplasmas, that is, small areas in which the breakdown voltage is less than that of the junction as a whole, must be eliminated, or at least minimized by using low dislocation materials, where appropriate, and by designing the active area to be no larger than necessary to accommodate the incident light beam (generally from a few micrometers to 100 micrometers in diameter).
Excessive leakage current due to high field concentration or junction curvature at the surface is eliminated by using a surface-contoured structure. See my prior patent U.S. Pat. No. 3,293,435 entitled "Semiconductor Charged Particle Detector", and Huth, et al U.S. Pat. No. 3,449,177 entitled "Radiation Detector", describing the surface contouring of a radiation detector, as well as Huth, et al U.S. Pat. No. 3,491,272 entitled "Semiconductor Devices With Increased Voltage Breakdown Characteristics" and Huth et al U.S. Pat. No. 3,575,644 entitled "Semiconductor Device With Double Positive Bevel", describing the beneficial effects of a positive bevel on the voltage breakdown characteristics of such devices as rectifiers. The teaching of U.S. Pat. Nos. 3,293,435, 3,449,177, 3,491,272 and 3,575,644 are incorporated herein by reference. In recent years, high power, high voltage thyristors (typically multilayered p-n-p-n devices) have been constructed that not only use bevelled surfaces to maximize the breakdown voltage, but that also use highly uniform n-type silicon obtained by a neutron transmutation doping process. See, in this regard, the discussion in Sze, supra, pp. 190-209.
Large area avalanche devices have been virtually impossible to construct because of what has been referred to as a base resistivity striation problem. As the silicon ingot is grown, dopant segregates in ridges at the growth interface, but not uniformly since the interface is a meniscus, giving rise to a resistivity fluctuation that can be as great as plus or minus 15-20%. The result is a "corrugated" electric field in the junction which prematurely breaks down at its weakest level and limits avalanche gains to about 50 to 100. As a result, avalanche devices have not been able to compete with or supplant image tubes where large area photoresponse is required. Image tubes referred to as "Digicons" make use of semiconductor electron detecting arrays to detect photoelectron images from a photocathode. Since the diode arrays themselves provide no gain, all the gain must be obtained by accelerating the photoelectrons under very high operating voltages, generally 15,000 to 30,000 volts. Such devices have the ability to detect even a single photoelectron but, because of the high voltage requirements, they have significant limitations in dynamic range, useful life, ease of manufacture and mechanical ruggedness, which severely limit their applications. Not only are such devices subject to problems such as arcing from the high acceleration potential, but radiation damage is so severe that spatial resolution degrades with time. This results in a drastic limitation of the number of photons that can be permitted to be incident, limiting such devices to astronomy applications and related uses.
Another type of device that uses an array is a charged coupled device, basically an array of closely spaced metaloxide-semiconductor ("MOS") diodes in which information represented by charge packets is serially transferred across a semiconductor substrate under the application of a sequence of clock voltage pulses. Such devices operate with no internal gain, although "on-chip" low noise FETs provide some amplification of the signal.